This invention generally relates to compositions and methods or processes particularly suited for gravel packing and workover of wells drilled in subterranean hydrocarbon-bearing formations, including without limitation horizontal and directional wells. This invention also generally relates to compositions employed as displacement fluids, packer fluids, and fluids for drilling, especially drilling through pay zones of subterranean hydrocarbon-bearing formations.
2. Description of Relevant Art
As used herein, the terms xe2x80x9cwell service fluidxe2x80x9d or xe2x80x9cwellbore fluid,xe2x80x9d unless indicated otherwise, shall be understood to mean a fluid used for sand control, gravel packing, and workover operations, as well as a fluid employed as a displacement fluid, a packer fluid, or a fluid for drilling and especially drilling through a pay zone. The term xe2x80x9csolids-freexe2x80x9d as applied to the basic well service fluid shall be understood to mean that no solid materials (e.g., weighting agents or commercial particulates) are present in the wellbore fluid (except that the term is not intended to exclude the presence of drill cuttings in the fluid in the well). The term xe2x80x9chorizontalxe2x80x9d with respect to a wellbore or to drilling shall be understood to mean at an angle or incline other than 90 degrees from the wellbore surface as the surface is viewed as a plane at ground level. The term xe2x80x9cbrine-sensitive formationxe2x80x9d shall be understood to mean a formation sensitive to brines or that is best suited for oil-based drilling and well service fluids, such as a formation having swelling or sloughing shales, salt, gypsum, anhydrite or other evaporite formation, a hydrogen-sulfide containing formation and hot (greater than 300 degrees Fahrenheit) holes.
A drilling fluid or mud is a specially designed fluid that is circulated through a wellbore as the wellbore is being drilled to facilitate the drilling operation. The various functions of a drilling fluid include removing drill cuttings from the wellbore, cooling and lubricating the drill bit, aiding in support of the drill pipe and drill bit, and providing a hydrostatic head to maintain the integrity of the wellbore walls and prevent well blowouts. Specific drilling fluid systems are selected to optimize a drilling operation in accordance with the characteristics of a particular geological formation.
A drilling fluid comprising liquid, usually water and/or oil or synthetic oil or other synthetic material or synthetic fluid (xe2x80x9csyntheticxe2x80x9d), with solids in suspension is typically called a drilling fluid or mud. A non-aqueous based drilling fluid typically contains oil or a synthetic as the continuous phase and may also contain water which is dispersed in the continuous phase by emulsification so that there is no distinct layer of water in the fluid. Oil or synthetic-based muds are normally used to drill swelling or sloughing shales, salt, gypsum, anhydrite or other evaporite formations, hydrogen sulfide-containing formations, and hot (greater than 300 degrees Fahrenheit) holes, but may be used in other holes penetrating a subterranean formation as well.
Gravel packing is becoming an increasingly common or favored method of completing a wellbore (as opposed to cementing and perforating a well). Well service fluids are used to carry the gravel into the well and place it at the gravel packing site. As used herein, the term xe2x80x9cgravel,xe2x80x9d shall be understood to include not only natural gravel but other proppant type materials, natural and man made or synthetic, such as, for example, sand, pebbles, and synthetic beads. Generally, the well""s bottom hole pressure, which must be equalized, determines the minimal density of the fluid needed.
A solids free water or brine, typically called xe2x80x9cclear brine,xe2x80x9d has been commonly used to carry gravel for gravel packing a well and for other well servicing operations. Such fluid typically contains a zinc or calcium or sodium halide, such as sodium chloride, dissolved therein to provide the desired density to the fluid.
However, when a well has been drilled with an oil or synthetic-based drilling fluid, xe2x80x9cclear brinexe2x80x9d fluids can be incompatible with the drilling fluid or with the subterranean formation having characteristics that necessitated the use of the oil or synthetic-based drilling fluid such as a brinexe2x80x94sensitive formation. Clean hydrocarbon oils would likely provide the most compatible or least damaging completion fluids, but such oils do not have the required densities and do not readily dissolve compounds that could provide the required densities.
A need exists for an oil or synthetic-based, solids-free, well service fluid that is compatible with oil or synthetic-based drilling fluids and brine-sensitive formations and that has the ability to be weighted to equal or greater than the bottomhole pressure requirements.
A well service fluid has been discovered that can be made compatible with oil or synthetic-based drilling fluids and brine sensitive formations. This fluid comprises a solids-free, invert emulsion comprising heavy brine, synthetic or oil, and an emulsifier sufficient for facilitating formation of an invert emulsion. The brine forms the internal phase of the emulsion and the synthetic forms the external phase of the emulsion.
Preferably, synthetic is selected that is compatible with the oil or synthetic drilling fluid used to drill the wellbore to be serviced. The brine is made heavy with halide salts, preferably calcium bromide or zinc bromide or mixtures thereof, or cesium formate. Such halides confer density to the emulsion fluid without need for addition of other materials or solids.